Recently, as a smartphone and a tablet PC are disseminated and high-capacity multimedia communication is activated, mobile traffic is rapidly increasing. It is expected that an increasing trend of the mobile traffic is going to be doubled in every year in the future. Since most of the mobile traffic is transmitted through an eNB, communication service providers are faced with a serious network load problem. Hence, the communication service providers has increases a network facility to process the increasing traffic and has hurriedly commercialized a next generation mobile communication standard capable of efficiently processing the huge amount of traffic such as WiMAX, LTE (Long Term Evolution), etc. However, in order to handle the traffic amount to be more increased, it is necessary to have a different solution.
In order to solve the aforementioned problem, study on D2D (device-to-device) communication is in progress. The D2D communication corresponds to a distributed communication technique for directly forwarding traffic between adjacent nodes without using such an infrastructure as an eNB. In D2D communication environment, a node (e.g., a mobile terminal, etc.) autonomously searches for a terminal physically adjacent to the node, establishes a communication session, and transmits traffic. In particular, since the D2D communication is able to solve a traffic overload problem by distributing traffic concentrated to an eNB, the D2D communication is getting a spotlight as an element technology of a next generation mobile communication technology after 4G. For this reason, a standard organization such as 3GPP (3rd generation partnership project) or IEEE (Institute of Electrical and Electronics Engineers) is trying to setup a D2D communication standard based on LTE-A (LTE-Advanced) or Wi-Fi.
It is expected that the D2D communication is going to contribute to performance enhancement of a mobile communication system and create a new communication service. And, the D2D communication may support such a service as an adjacency-based social network service or a network game. The D2D communication may solve a connectivity problem of a terminal located at a radio shadow area by utilizing a D2D link as a relay. In particular, it is anticipated that the D2D technology will provide a new service in various fields.
Meanwhile, a device-to-device technology such as infrared communication, ZigBee, RFID (radio frequency identification), NFC (near field communications) based on the RFID, etc. is already widely used. Yet, since the abovementioned technologies support communication of a special purpose only within a very constrained distance (1 m or less), it is difficult to categorize the technologies as a D2D communication technology that distributes traffic of an eNB.
Meanwhile, since a terminal has a limited battery capacity in general, a transmission terminal may transmit a signal with low power. If a signal is transmitted with low power, it may reduce link reliability. As an alternative, MIMO (Multiple-Input and Multiple-Output) technique can be applied to the D2D communication. Yet, a method of performing D2D communication to which the MIMO technique is applied has not been proposed yet.